Remove unnecessary friend decls; fix Win-Clang builds

src/register-configuration.cc

Index: src/register-configuration.cc
diff --git a/src/register-configuration.cc b/src/register-configuration.cc
index 9b1c951f83849dd8951e586e39547fca8d5ad133..9773c2ac5798fdc16d7e1cc686f647102a4a6ae5 100644
--- a/src/register-configuration.cc
+++ b/src/register-configuration.cc
@@ -82,9 +82,6 @@ class ArchDefaultRegisterConfiguration : public RegisterConfiguration {
   const char* double_register_name_table_[DoubleRegister::kMaxNumRegisters];
 
  private:
-  friend struct Register;
-  friend struct DoubleRegister;
-
   static const int* GetAllocatableGeneralCodes() {
 #define REGISTER_CODE(R) Register::kCode_##R,
     static const int general_codes[] = {